Here is a link to an MSDS. (google GaAs MSDS)
I wouldn't worry about handling of bulk material, but turning, or grinding would need ventilation and fume extraction. Also, the waste needs to be treated as toxic material. (coolant, etc)
I believe that it also dissociates into Gallium and Arsenic if burned.
On a more practical level, it's also very brittle.
I work with GaAs wafers, but I'm not a safety guy, so I'm sure there's a lot more you need to know.